1. Field of the Invention
This invention relates generally to the field of liquid cleaning baths and, in one particular embodiment, to a method and device for evaluating and/or adjusting the cleaning performance of an aqueous cleaning liquid in a cleaning bath of a production process, such as an automotive production process.
2. Technical Considerations
In many production processes, parts are fabricated, cleaned, coated, and assembled into a final product. For example, in the appliance field, metal frames for appliances, such as refrigerators, stoves, washers, dryers, and the like, are shaped and painted before final assembly. As a further example, in a conventional automotive production process, the individual parts and components used to make up a vehicle are initially machined, welded, or fabricated at a body shop. In the course of this process, protective oils and lubricating oils are applied to the parts to aid in cutting and forming the parts and also to prevent corrosion. The “part” could be an assembled auto body, conventionally referred to as a “body in white”, or could be one or more smaller parts or pieces. From the body shop, the parts are transferred to a paint shop for the application of various coatings, such as anticorrosion coatings and color coatings. However, prior to coating, the parts must be thoroughly cleaned and degreased to remove the oils and grime accumulated in the body shop to ensure that the applied coatings will cover and adhere to the parts evenly. This cleaning operation can involve one or more spray cleaning steps and one or more immersion cleaning steps in which the parts are respectively sprayed with or immersed in a cleaning liquid. The cleaning liquid is typically an aqueous surfactant solution. After the parts are cleaned and coated at the paint shop, they are transferred to an assembly shop to be assembled into the vehicle. Because it is cost intensive to operate a production line, it is desirable that the line not be stopped during scheduled production time. Every effort is made to ensure that process tank chemicals perform adequately until they can be conveniently changed on scheduled maintenance days.
In the course of this conventional production process, the cleaning performance of the cleaning liquid will eventually decrease due to such factors as oil loading and bath drag out. By “oil loading” is meant the build-up of oil and dirt in the cleaning liquid from the parts being cleaned. The cleaning bath can become saturated with oil and dirt to the point where only marginal levels of cleaning agent, e.g., surfactant, are available for cleaning. By “bath drag out” is meant the loss of cleaning liquid (both solvent and surfactant) carried out of the bath by adherence to the cleaned parts. If no action is taken, the cleaning performance of the cleaning liquid will eventually degrade to the point where the parts are not adequately cleaned of oils and grime prior to application of the coatings. In which case, the coatings may not adhere to oily or dirty portions of the part or may be unevenly distributed on the part due to the presence of oil and/or dirt. Such poorly coated parts oftentimes must be either discarded or sanded down and re-coated, which can increase production time as well as cost.
In order to avoid this problem, the cleaning liquid may be preemptively disposed of, or additional cleaning liquid may be added to the bath to recover cleanability, or the bath may be operated at a higher cleaning agent concentration than normally needed out of fear of losing cleaning performance. Most conventional automotive production facilities simply dump the cleaning liquid and replace it with fresh cleaning liquid after a set period of time before the cleaning liquid can degrade to the point when the cleaning performance becomes unacceptable. The time period to dump and replace the cleaning liquid is typically determined by prior cleaning experience or by comparison with the cleaning baths of other production facilities. For example, if prior experience has taught that the cleaning performance of a particular cleaning liquid at one production facility typically degrades to unacceptable levels after about two and a half weeks, the cleaning liquid may be dumped after only about two weeks just to avoid the possibility of inadequately cleaned parts. While this procedure does decrease the occurrence of poorly cleaned and, hence, poorly coated parts, it can also lead to the premature dumping of cleaning liquid which could still be perfectly adequate for cleaning, i.e. which still has acceptable cleaning performance. Also, just because a cleaning liquid may degrade to unacceptable levels at one facility in a particular period of time does not necessarily mean that the same or different cleaning liquid will degrade in the same time period at another facility. This premature dumping of cleaning liquid can increase production costs since usable cleaning liquid could be, and oftentimes is, prematurely dumped and replaced with fresh cleaning liquid.
Rather than simply dumping the cleaning liquid after a given time period or operating at excessive cleaning agent concentrations, it would be advantageous if the cleaning liquid could be easily and economically tested on-line to determine whether the cleaning performance was still adequate or whether the cleaning performance was approaching the point of unacceptability. While the amount of oil in the cleaning liquid could be analytically measured, such a procedure would be prohibitively time consuming and complicated for most conventional industrial applications. Conventional surface tension measurements, such as capillary rise, du Nouy ring, and Wilhelmy plate methods, measure static surface tension and are not easily adapted to measure dynamic changes to the surface tension of an on-line cleaning liquid. Also, these conventional methods may be appreciably influenced by the state of wetting and the contact angles between the solution and the ring or glass surface.
One commonly used method of estimating the cleaning performance of a conventional cleaning liquid is by measuring the alkalinity of the cleaning liquid. In many conventional cleaning systems, the cleaning liquid includes not only surfactants but also alkaline builders. By “builders” is meant the inorganic salts used to soften the water and/or change the structure of the water to enhance surfactant performance. Examples of such builders include alkali metal salts of silicates, carbonates, and phosphates. It is assumed that the surfactants are consumed at about the same rate as the alkaline builders and, hence, the surfactant level is estimated from the amount of alkaline builders remaining in the bath. However, while the surfactant concentration can be loosely correlated to the alkalinity of the coating liquid, this is in reality simply an indirect measurement and may not be particularly accurate for any one particular cleaning liquid. Additionally, while this estimation process can be utilized for conventional alkaline cleaning liquids, it cannot be used for cleaning baths incorporating bioremediation. As will be appreciated by one skilled in the art, “bioremediation” refers to the presence of oil-consuming bacteria in the cleaning liquid to break down oils in the cleaning bath. In bioremediation systems, conventional alkaline builders are typically not used or not used in any great quantity since such alkaline builders tend to kill the bacteria.
Therefore, it would be advantageous to provide a method and/or device that could be easily and economically utilized to predict or measure the cleaning performance, e.g., cleaning agent or surfactant level or concentration, of a cleaning liquid, such as in an automotive production process. It would further be advantageous to provide a method and device for adjusting the cleaning performance of a cleaning liquid when the measured cleaning performance is at or below a desired level.